It is true that the US could delete ".ir" from the current DNS root servers, but any DNS administrator worth his salt could work around this in a few minutes.
It is very easy to set up replacement root servers anywhere in the world and edit routing tables to direct requests to them, cutting the US servers out of the loop. This could easily be done in a day or less and would require only one small change at most ISPs to enable it for their customers.
In fact, any company or ISP can bring ".ir" back online for their employees/customers in under 15 minutes by simply doing a zone transfer from the root servers to their own DNS servers, then adding in the deleted records.
This is obvious to any network administrator, so I would be very surprised if the US ever tried such a thing. Iran would be online again in a day or so, and the negative publicity cost to the US would be significant.
These facts should be obvious to those making the claim, so we should look elsewhere for their motives.